Preferably, the plane of an object to be photographed or viewed should be essentially perpendicular to the optical axis of the camera or viewing system; and the image plane should be perpendicular to the same optical axis. In a variety of applications, though, this preferred geometry can be maintained only with great difficulty and inconvenience. Examples would be photographing tall buildings from ground level or viewing objects from a small oblique angle to the object plane, where the object plane is tilted considerably relative to both the optical axis and the image plane. Acceptable focus of all parts of the image may not be attainable.
An early solution to this type of problem is found in German Patent 164,527, granted to Theodor Scheimpflug, who discovered that well-focused images could be obtained under such circumstances if the object and image planes were made to intersect with plane of the optical system (lens) on a common straight line. The general arrangement of a Scheimpflug viewing system is shown in FIG. 1. An obliquely tilted object plane 10 is viewed through a first optical system such as a lens 12, which has an axis 14 forming an oblique angle with object plane 10. The plane 16 of lens 12 is essentially perpendicular to axis 14 in the illustrated system but could also be at an oblique angle. An image plane 18 forms an angle of incidence .alpha. between axis 14 and a normal to the image plane. Scheimpflug taught that a good image could be formed at image plane 18, provided that object plane 10, lens plane 16 and image plane 18 intersect along a common line 20. Thus, photographic film placed at image plane 18 could be exposed to produce a good image of an object at plane 10. Photographic film is especially suited for use in Scheimpflug systems since film can be made large enough to accommodate most images and will capture images using incident light at very large angles of incidence approaching parallel to the surface of the film.
In various more modem vision systems, however, real time observation of an object is desired, such as the condition of a product or machine pan during performance of an industrial process. Often, access to the object to be viewed is rather severely limited, which may require use of viewing angles of only several degrees from the plane defined by the object. While a Scheimpflug system can be applied in such situations with acceptably good results when conventional photographic film is exposed at the to image plane, difficulties arise when real time measurements are desired using an electronic detector such as a CCD array or the like, including camera tubes. Such detectors typically are rather small, making it difficult to capture an entire image of the object. In addition, such detectors rather typically are recessed within a camera housing or an integrated circuit housing, thus requiring a rather small angle of incidence (45.degree. maximum) or an unusually large, very expensive detector to provide sufficient area to receive light from the object. Furthermore, optical anti-reflection coatings commonly used on such detectors are generally designed for nearly normal incident light, not very oblique light such as would be received in systems similar to that of FIG. 1.
Thus, a need has existed for a simple, inexpensive device for redirecting light received from the object at the image plane in a Scheimpflug type system; so that the light can be relayed to an electronic detector along an axis more nearly normal to the surface of the detector.